Swimming pools without supervision present a danger particularly for children having free access to the sides of the swimming pools.
Every year the number of victims of accidental drowning in swimming pools are to be deplored, especially in family swimming pools, whether such drowning is caused by an accidental fall of the child or by deliberate immersion when adults are not in the immediate vicinity.
The need has then made itself felt of providing supervision of swimming pools by reliable means outside authorized bathing periods during which children are supposed to be supervised by adults.
The apparatus concerned comprises a case, a membrane and a piezo-electric cell for detecting frequency waves less than or equal to 5 Hz and connected to a signal amplification and management circuit. In fact, a fall in water gives rise to sound or subsonic waves whose frequency spectrum is limited to the range less than or equal to 5 Hz.
It is therefore desirable for the detector to take into account exclusively the frequency band mentioned so as to exclude as completely as possible all the frequencies which may be transmitted to the detector from other sources than persons falling in water.
Detectors are known for detecting the fall of objects in water, in which elimination of the electric signals due to disturbing frequencies not representative of falls in water is provided by means of a electronic filtering circuits.